The disclosure of Japanese Patent Application No. 2000-67916 filed on Mar. 13, 2000 is incorporated herein by reference in its entirety.
1. Field of the Invention
This invention relates to a wire breaking detection method for a temperature sensor and a temperature sensing apparatus for using the method.
2. Description of the Related Art
In an electric power steering apparatus for an automobile, steering operation is assisted by driving a motor according to steering torque that is measured by a torque sensor. When the motor outputs continuously a large amount of power, temperature of the motor turns up. It causes deterioration of performance of the motor. To avoid the deterioration of the motor, when high temperature is sensed by a temperature sensor attached to the motor, a warning signal such as a lamp is turned on. A temperature sensor consisted with a thermal resistor (hereinafter xe2x80x9cthermistorxe2x80x9d) and a pull-up resistor is generally known as above described temperature sensor. In this temperature sensor, variations of resistance of the thermistor are measured as variations of voltage, and the temperature can be determined by the variations of voltage. The pull-up resistor that has dozens kxcexa9 resistances is connected series with the thermistor, and voltages, for example five volts, are applied to them. Then the temperature can be determined by measuring voltages that acts to the thermistor.
Since the thermistor is weak in vibrations, when the temperature sensor is attached on the automobile, it is possible that a wire breaking of the thermistor happens by vibrations of the automobile.
In a case of the temperature sensor is adapted for the motor of the electric power steering apparatus, since a temperature range of an operating area for the motor is wide such as from xe2x88x9240xc2x0 C. (in a case of using the automobile in a cold district) to 100xc2x0 C. (highest temperature of permissible tolerance limits for using of the motor), resistance of the thermistor varies widely such as from several Mxcexa9 (xe2x88x9240xc2x0 C.) to dozens kxcexa9 (100xc2x0 C.). In low temperature, resistance of the thermistor becomes very high quantity such as several Mxcexa9. Therefore, from the amount of measuring voltage only, it can not be distinguished a wire breaking of the thermistor from high value of resistance caused by low temperature. For example, when five volts voltages are applied to the thermistor and the pull-up resistor having dozens kxcexa9 resistances, since measured voltages that acts to the thermistor are approximately five volts in both case of the thermistor having several Mxcexa9 resistances by low temperature such as xe2x88x9240xc2x0 C. and having infinity resistance by a wire breaking of the thermistor, it can not be distinguished a wire breaking of the thermistor from low temperature.
In view of the foregoing, an object of the present invention is to provide a wire breaking detection method for a temperature sensor and a temperature sensing apparatus for using the method, which can detect a wire breaking even in low temperature.
In order to achieve the above object, the present invention provides the wire breaking detection method for a temperature sensor that measures temperature of an electric driven device according to a variation of a resistance thereof. In this method, whether a predetermined quantity of current is applied to the electric driven device during a predetermined time is judged. When the predetermined quantity of current is applied to the electric driven device during the predetermined time, a measured temperature, which is measured by the temperature sensor, is comparing with a predetermined temperature. And, when the measured temperature is lower than the predetermined temperature, it is decided that a wire of the temperature sensor breaks.
In order to achieve the above object, the present invention provides the temperature sensing apparatus for measuring temperature of an electric driven device. The temperature sensing apparatus comprises a temperature sensor, a current measuring device and a temperature detection circuit. The temperature sensor outputs equivalents of resistance of a thermal resistor that is contained therein and disposed near the electric driven device. The current measuring device measures quantity of current that is applied to the electric driven device. The temperature detection circuit detects temperature of the electric driven device according to the output equivalents from the temperature sensor. And, the temperature detection circuit judges whether a predetermined quantity of current is applied to the electric driven device during a predetermined time according to quantity of current measured by the current measuring device. When the predetermined quantity of current is applied to the electric driven device during the predetermined time, the temperature detection circuit compares a temperature detected by the current measuring detection circuit with a predetermined temperature. When the detected temperature is lower than the predetermined temperature, the temperature detection circuit decides a wire of the thermal resistor breaks.
According to the present invention, the wire breaking is detected by comparing the measured temperature with the predetermined temperature when the predetermined quantity of current is applied to the electric driven device during the predetermined time. Therefore, the wire breaking can be detected even in low temperature.